One interesting way to look at this is to do the math and chart not just the decline in homicide but the even steadier and more amazing decline in unsolved homicide. Part of the story could just be that the cops are getting smarter. One possible answer is that as crime falls the remaining crimes are disproportionately easy ones to solve—perhaps domestic violence cases. But my guess is the larger issue is simply that when the MPDC has fewer homicides to investigate it's able to investigate the remaining ones more thoroughly and so it gets more difficult to get away with murder. A virtuous circle, in other words, where the decline of an atmospheric toxin not only makes us safer but makes our public safety services more effective in coping with the remaining problems.